Baseline laboratory parameters for preliminary diagnosis of COVID-19 among children: a cross-sectional study

ABSTRACT BACKGROUND: Clinical judgment of initial baseline laboratory tests plays an important role in triage and preliminary diagnosis among coronavirus disease 2019 (COVID-19) patients. OBJECTIVES: To determine the differences in laboratory parameters between COVID-19 and COVID-like patients, and between COVID-19 and healthy children. Additionally, to ascertain whether healthy children or patients with COVID-like symptoms would form a better control group. DESIGN AND SETTING: Cross-sectional study at the Institute for Child and Youth Health Care of Vojvodina, Novi Sad, Serbia. METHODS: A retrospective study was conducted on 42 pediatric patients of both sexes with COVID-19. Hematological parameters (white blood cell count, absolute lymphocyte count and platelet count) and biochemical parameters (natremia, kalemia, chloremia, aspartate aminotransferase [AST], alanine aminotransferase [ALT], lactate dehydrogenase [LDH] and C-reactive protein [CRP]) were collected. The first control group was formed by 80 healthy children and the second control group was formed by 55 pediatric patients with COVID-like symptoms. RESULTS: Leukocytosis, lymphopenia, thrombocytosis, elevated systemic inflammatory index and neutrophil-lymphocyte ratio, hyponatremia, hypochloremia and elevated levels of AST, ALT, LDH and CRP were present in COVID patients, in comparison with healthy controls, while in comparison with COVID-like controls only lymphopenia was determined. CONCLUSIONS: The presence of leukocytosis, lymphopenia, thrombocytosis, elevated systemic inflammatory index and neutrophil-lymphocyte ratio, hyponatremia, hypochloremia and elevated levels of AST, ALT, LDH and CRP may help healthcare providers in early identification of COVID-19 patients. Healthy controls were superior to COVID-like controls since they provided better insight into the laboratory characteristics of children with novel betacoronavirus (SARS-CoV-2) infection.

early diagnosis. Thrombocytosis is another common laboratory finding. 7 Hematological indices such as the systemic inflammatory index (SII), neutrophil-lymphocyte ratio (NLR) and platelet-lymphocyte ratio (PLR) are useful biomarkers for assessments of disease severity and prognosis among patients with pneumonia. 8 Electrolyte disturbances such as hyponatremia, hypokalemia and hypochloremia have been corelated with COVID-19 infection. 9 Even in mild cases of COVID-19, hepatic transaminase levels (alanine aminotransferase [ALT] and aspartate aminotransferase [AST]) may be elevated due to transient liver damage. 10 Lactate dehydrogenase (LDH) is an enzyme associated with tissue damage and is a biomarker of interest in COVID-19 patients. 11,12 C-reactive protein (CRP) is an important clinical biomarker of inflammation and infection. Altered CRP levels may be linked to the degree of disease severity among COVID-19 patients. 13 All of these data were primarily documented from adult cases.
The incidence of manifest COVID-19 in the pediatric population is significantly lower than the incidence of infected adults. 8 Therefore, information about hematological and biochemical parameter alterations in children with COVID-19 is very limited worldwide. Furthermore, the interpretation of these results varies to a significant extent. 8,14 OBJECTIVE The objective of this study was to determine the differences in laboratory parameters between COVID-19 and COVID-like patients, and between COVID-19 patients and healthy children.
Additionally, we aimed to find out whether healthy children or patients with COVID-like symptoms would form a better control group.

METHODS
A retrospective study was conducted on 42 pediatric patients of both sexes with COVID-19. All of these patients were admitted to the confirmed-infection isolation wards at the Institute for Child and Youth Health Care of Vojvodina between April 2020 and January 2021. The sample size was not calculated because we had 42 COVID cases in total within the abovementioned period and we included all of them in the study. Occurrences of SARS-CoV-2 infection were confirmed through RT-PCR, performed on nasopharyngeal and throat swab specimens from the patients.
The hematological and biochemical findings from blood samples collected on the day of admission were recorded. The patients did not receive any therapy before blood collection. The hematological values were tested using the Advia 2120 hematology analyzer (Siemens Healthcare, Germany), for complete blood counts with a differential white blood cell count. The following hematological results were collected: white blood cell count, absolute lymphocyte count and platelet count. Additionally, SII, NLR and PLR were calculated. The biochemical values were tested using the AU 480 chemistry analyzer (Beckman Coulter, Switzerland). The following biochemical results were collected: natremia, kalemia, chloremia, AST, ALT, LDH and CRP.
For comparative analyses, two control groups of age and sexmatched SARS-CoV-2-negative patients were enrolled in the study. The first control group was formed by 80 children who were healthy at that moment and had come to our institution for their regular check-ups. A total of 55 pediatric patients with COVID-like symptoms formed the second control group. Patients with malignancy were excluded from our analysis. All the controls were tested negative for COVID-19 prior to admission by means of a rapid test for

RESULTS
Between April 2020 and January 2021, a total of 42 pediatric cases of COVID-19 infection were admitted to the Institute for Child and Youth Health Care of Vojvodina. These patients' average age was 5.24 ± 6.04 years. The female share of the group was 57.1%, with an average age of 6.12 ± 6.54 years; and 42.9% of the group were males with an average age of 4.07 ± 5.25 years. The first control group consisted of 80 healthy children with an average age of 5.84 ± 6.12 years. The female share of this group was 41.3%, with an average age of 6.63 ± 6.46 years; and 58.7% of the group were males with an average age of 5.28 ± 5.89 years. The second control group was formed by 55 children with COVID-like symptoms, with an average age of 5.39 ± 5.81 years. The female share of this group was 49.1%, with an average age of 5.38 ± 5.98 years; and 50.9% of the group were males with an average age of 5.40 ± 5.73 years. The composition of the groups according to age and sex is presented in Table 1.
Binary logistic regression was used to determine that age was  (Table 4).
Furthermore, statistically significant laboratory parameters were studied through receiver operating characteristic (ROC) analyses. Only laboratory markers with an area under the curve (AUC) above 0.7 were considered acceptable for analyzing the exact cutoff value: CRP (AUC: 0.842), AST (AUC: 0.779) and LDH (AUC: 0.712) (Figure 1)     Additionally, we analyzed the differences between the COVID-19 group and the healthy children according to the cutoff values for CRP, AST and LDH. These differences were statistically significant for all three parameters ( Table 5).

DISCUSSION
Considering the global epidemiology and alarming severity of COVID-19 infection, well-timed diagnostics are crucial. 15 The turnaround time for RT-PCR results is supposed to be very quick, but the lack of human and laboratory resources in many countries has led to delays in SARS-CoV-2 confirmation. During this period, baseline hematological and biochemical analyses are essential for enabling a clinical judgement when COVID-19 infection is suspected. 15,16 In the initial analyses, in which patients were compared with healthy controls, we observed leukocytosis, lymphopenia, thrombocytosis, elevated SII and NLR, hyponatremia, hypochloremia and elevated levels of AST, ALT, LDH and CRP. However, comparison of patients with COVID-19 and patients with COVID-like symptoms showed that only lymphopenia might play an important role in distinguishing patients within these two groups. In a review paper published by Lippi 19 Thus, it can be said that the laboratory findings from different clinical and research centers vary to a significant extent.
Several factors may have influenced this discrepancy in the laboratory results. In the first place, the medical community and general public have had a constant need for new information about the COVID-19 pandemic, and this has been paramount. The methodological quality of published reports has been lower than that of similar studies published prior to the pandemic. 20 Furthermore, this need to disseminate information promptly has been forcing researchers to opt for simpler study designs. Even in some major journals, many observational studies have been published without control groups. 21 Additionally, selecting an appropriate comparison group is crucial. Use of more than one control group has often been discussed in observational studies. When healthy controls are used, they are expected to show any laboratory distinction between COVID-19 patients and healthy children. On the other hand, when patients with COVID-like symptoms are used as controls, it is possible to determine which laboratory parameters might be specific for SARS-CoV-2 infection, regardless of the symptoms. 22 In our study, no specific laboratory parameters were deter-    Elevated aminotransferase levels are a common biochemical abnormality in COVID-19 patients, most probably due to liver injury associated with the immune response. However, except in the sense of being another diagnostic marker, their prognostic significance still remains uncertain. 10 The serum concentration of LDH is an important marker of tissue damage and its elevation has been correlated with worse outcomes in cases of viral infections in general. 11 Moreover, because of inflammatory reactions and tissue destruction, CRP levels in SARS-CoV-2-positive children tend to increase significantly. 27 This study had some limitations. The first limitation was the small number of participants, given that this was a single-center study and that many children affected by the virus have no symptoms. Nonetheless, our findings could provide the basis for further research. The second limitation was that inflammation-related biomarkers, such as procalcitonin, interleukin-6 and presepsin were not included. Nor were hemostasis biomarkers such as partial thromboplastin clotting time (PTT), activated partial thromboplastin clotting time (aPTT), fibrinogen and D-dimer.
Unfortunately, no such data were available for all the patients with confirmed SARS-CoV-2 infection because such analyses do not form part of the standard diagnostic protocol upon admission to our Institute, and are performed on demand only. Therefore, the baseline laboratory parameters were the main focus of interest in our study.

CONCLUSION
All the laboratory markers mentioned above may help healthcare providers in early identification of COVID-19 patients.
All of these parameters may be used for developing novel diagnostic scores for pediatric COVID-19 patients. CRP, AST and LDH demonstrated the best diagnostic performances, considering their sensitivity and specificity. Based on this study, it can additionally be concluded that healthy controls are superior to COVID-like controls since they provided better insight into the laboratory characteristics of children with SARS-CoV-2 infection.